The present invention relates to the field of motion detection, and particularly, motion detection using passive infrared sensors.
Objects that generate heat, such as the human body, also generate infrared radiation. Although infrared radiation exists at a wavelength that is longer than visible light, devices exist for detecting infrared radiation. One such device is the pyroelectric sensor. The pyroelectric sensor is also referred to herein as the passive infrared (PIR) sensor. The pyroelectric sensor includes a pyroelectric element made of a crystalline material that generates a surface electric charge when exposed to heat in the form of electromagnetic radiation. When the amount of radiation striking the crystal changes, the amount of charge also changes. This change in the amount of charge is then measured. When a significant change in charge occurs, the sensor generates a signal representative of the change. Because the sensor is generally capable of detecting radiation over a wide frequency range, a filter window is added to the sensor to limit the incoming radiation to a certain desired frequency range. For example, the filter window may limit incoming radiation to a range that is most sensitive to the human body (e.g., 8 to 14 μm).
One type of PIR sensor is a sensor having two pyroelectric elements configured to produce voltages of opposite polarity when infrared radiation is detected. This arrangement with two detectors having opposing signals causes signals resulting from vibration, temperature changes and sunlight to be cancelled by the sensor, since in those situations, both pyroelectric elements fire at the same time. If the pyroelectric elements in such a PIR sensor are arranged in a horizontal plane, a warm body must pass horizontally in front of the sensor for an output to result. When a warm body does pass horizontally in front of the sensor, the first pyroelectric element will trigger and then the second, resulting in an output having a positive (or negative) voltage pulse immediately followed by a voltage pulse in the opposite direction.
PIR sensors have been used for years in various applications. Perhaps the most common application is the use of the PIR sensor in security systems. For example, PIR sensors have been used to detect movement through a detection zone and the direction of movement through the detection zone. An exemplary arrangement for such a motion detection system using a PIR sensor is shown in FIG. 1. As shown in FIG. 1, a PIR detector 100 having a wide field of view is focused by a fresnel lens into two diverging detection zones. A first detection zone 104 is shown in FIG. 1 by dotted lines 110 and 112. A second detection zone 106 is shown by dotted lines 114 and 116. Although the fresnel lens 102 provides some focusing of the detection zones, the two detection zones 104 and 106 typically remain several feet removed in the area where a human would be detected. When a warm body 108, such as a human, enters the first detection zone 104, a first pyroelectric element 103 monitoring that detection zone indicates a change in infrared radiation in that zone, and a positive signal is output from the sensor 100. When the warm body enters the second detection zone 106, a second pyroelectric element 105 monitoring that detection zone indicates a change in infrared radiation in that zone, and a negative signal is output from the sensor 100. The direction of movement through the detection zones can be determined by noting whether the leading edge of the output signal was positive, indication the first pyroelectric element fired first, or negative, indicating the second pyroelectric element fired first. This arrangement is generally useful for detecting the presence of a warm body in the detection zones and/or the direction of movement through the detection zones. However, because the detection zones are several feet apart and not focused about a specific line, these arrangements are not useful in determining a precise moment that the warm body crossed a particular line.
Another arrangement for PIR sensors in home security applications includes the use of a PIR sensor as a general motion detector for a room. In these applications, a PIR sensor is often mounted in the corner of a room and operable to detect motion of a warm object within the room. These detectors typically include a single PIR sensor with a wide field of view and a lens having numerous facets that split the field of view into numerous corresponding detection sections with small “blind spots” between the sections. When the PIR sensor detects a change in infrared light between facets, the motion detector provides a signal to indicate movement in the room. As with other PIR sensor arrangements, these PIR sensors have proved adequate for motion detection over a wide area, but have not been useful for detection of motion through a well defined line or boundary.
Motion through a well defined line has typically been detected using a beam of light such as a laser. These “through beam” lasers transmit a stream of light across a distance, with the laser defining the boundary to be monitored. When the through beam laser is broken, a receiver opposite the light transmitter detects that the light beam has been broken. Alternatively, when the light beam is broken, a receiver next to the transmitter will pick up reflected light and detect that the light beam has been broken. Unfortunately, these “through beam” detectors have several disadvantages. First, they tend to consume a large amount of energy. In order to reduce energy consumption, the through beam may be periodically pulsed, but such a pulse could possibly result in a breach of the line without detection if the period of the pulse is significant. Second, through beam detectors are costly because both a light transmitter and a receiver are required. Third, if the receiver is to be mounted opposite the transmitter, housings for the detection system must be located in different areas of a space, making the existence of the detector more noticeable.
PIR sensors are typically inexpensive and operate on very low energy requirements. Accordingly, a PIR sensor that could be used to detect motion through a well defined line or boundary would be useful in a number of human monitoring applications. Such a detector would be useful for generally detecting movement of a human across a defined boundary. Furthermore, such detectors could be useful in noting the precise moment when an individual crossed a boundary, and if used in combination, the average rate of speed of an individual over a defined distance.